Fix ERC2771ContextMock for upgradeable contracts

contracts/mocks/ERC2771ContextMock.sol

@@ -7,6 +7,7 @@ import "../metatx/ERC2771Context.sol";
 
 // By inheriting from ERC2771Context, Context's internal functions are overridden automatically
 contract ERC2771ContextMock is ContextMock, ERC2771Context {
+    /// @custom:oz-upgrades-unsafe-allow constructor
     constructor(address trustedForwarder) ERC2771Context(trustedForwarder) {
         emit Sender(_msgSender()); // _msgSender() should be accessible during construction
     }